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today first encounter const variable defined, access to relevant information put together this article. The main contents are: js three definitions of the variables in the way const, the difference between var, let's.
1.const defined variables can not be modified, and must be initialized.
const b = 2; // correct // const b; // error must be initialized console.log ( 'outer const function definition b:' + b); // the output value // B =. 5; // Console. log ( 'const modify definitions outside the function b:' + b); // can not be output
2.var defined variables can be modified, if not initialize prints undefined, does not complain.
A. 1 = var;
// var A; // no error
console.log ( 'function definition outer var a:' + a); // can be output. 1 = A
function Change () {
A =. 4;
the console.log ( 'the function defined var a:' + a); // can be output. 4 = a
}
Change ();
the console.log ( 'after a function call is defined as a function of the internal var modified value:' + a); // can output a = 4
3.let block-level scope is, let the internal function definitions used, no effect on the external function.
C =. 3 let;
the console.log ( 'external function definition let c:' + c); // output =. 3 C
function Change () {
let C =. 6;
the console.log ( 'let the function definitions c:' + c); // output =. 6 c
}
Change ();
the console.log ( 'defines the internal function is not affected by the definition of c let the function call:' + c); // output c = 3
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Disclaimer: This article is the original article CSDN bloggers "cicada sitting up" and follow CC 4.0 BY-SA copyright agreement, reproduced, please attach the original source link and this statement.
Original link: https: //blog.csdn.net/qq_36784628/article/details/80966826